Towed acoustic transmitter

ABSTRACT

The invention relates to towed acoustic transmitters forming an underwater vehicle which is itself intended for towing a linear acoustic receiving array (105). It consists in placing the vertical faired acoustic array (303) of such a transmitter at the very rear of the underwater vehicle, and in balancing the weight of this array by a faired ballast (313) situated at the very front thereof. These two parts are joined by a girder (310) of small cross-section, and the center of gravity of the whole is situated at the front of this girder and beneath it. The vehicle is towed by a cable (101) fastened to the vehicle by a hook (314) fixed to a swivel joint (315) above the center of gravity. It makes it possible to facilitate the operations of submersion and fishing-out of the whole while also achieving a greater depth of submersion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to towed acoustic transmitters which makeit possible to transmit acoustic signals in seawater which are intendedin particular to be received, after reflection on a detected obstacle,by a linear acoustic receiving array which is itself towed astern ofthis transmitter.

2. Description of the Related Art

It is known to tow behind a boat an underwater vehicle which may containvarious payloads such as a sonar. This vehicle sails astern of thetowboat at a submersion determined for example by flaps which aremanoeuvred appropriately so as to maintain this submersion at thedesired value.

A particular case consists, as represented in FIG. 1, in towing behind ahauling boat 101, with the aid of a towing cable 102, a vehicle 103essentially comprising an acoustic transmitter which makes it possibleto sweep the underwater volume with sound. The return echoes obtainedthrough this sound-sweeping are received by an acoustic linear array105, which is itself towed by a cable 104 fastened astern of theunderwater vehicle 103. This kind of device being intended to operate atrelatively low frequencies, of the order of a kHz, the transmittercontained in the vehicle 103 is therefore relatively voluminous. Toobtain the directivity characteristics which are generally desired, inparticular in the vertical plane so as to combat the phenomena ofreverberation, this acoustic transmitter is generally of elongate shape,with a height of around 4 to 5 times its thickness as well as its width.

In order to be able to maintain these directivity characteristics, it isof course necessary for the transmitting array, and hence the body ofthe vehicle to which it is generally rigidly attached, to move along arectilinear trajectory, at the very least while the boat is not turning,and to maintain a constant trim and constant heading, that is to say inpractice for the body to remain vertical and oriented towards the boat.To achieve this, use is generally made of a structure such asrepresented in FIG. 2. The body 203 of the vehicle, which hassubstantially the shape of a column with elliptical cross-section offairly large aspect ratio, is hauled by the cable 102, to which it isjoined by a rigid stirrup 202, whose points of fixing to the vehicle arefixed in an articulated manner at the location of the centre of drag ofthis body. In this way this stirrup ensures good roll stability of thehauled vehicle. The cable 104 for towing the linear array 105 is itselftied to the rear of the body, at a spot situated in the horizontal planepassing through this centre of drag. To ensure the vertical stability ofthe vehicle, the distribution of the masses inside the body is contrivedin such a way that the centre of gravity 101 of the latter is locatedwell beneath the articulation between the stirrup 202 and the body 203.The yaw stability originates in part from the action of the stirrup 202and it is improved by using vertical fins 205 fixed to the top andbottom of the body.

Such a device operates relatively well, at the very least while theheight/width ratio of the body of the vehicle is not too large and it isnot sought to submerge it too deeply.

Now, the current tendency consists in seeking deeper and deepersubmersions both in respect of the transmitter and in respect of thereceiving array associated therewith, essentially so as to increase thedetection range. Under these conditions, and since the length of thehauling cable 102 cannot be permitted to be excessively lengthened, forlarge depths an inclination of the stirrup 202 with respect to thevertical of the body of the vehicle is obtained which is itself large.Bringing the stirrup closer to this vertical tends to increase theinstability of the body of the towed vehicle considerably.

Furthermore, in order to have simultaneously a higher array gain andlower electrical consumption, it is also sought to use much tallertransmitters than those already known. The vehicles comprising thesetransmitters then have substantially the shape of a vertical wing oflarge height/width ratio. Such vehicles have very poor stability in yawand in the event of the manoeuvring of the hauling boat, thus tending tomake them drift from a rectilinear trajectory. Furthermore, as soon asthe speed exceeds about ten knots the appearance of an effect of the"kite" type is observed. Under this effect the body pulls to one side orthe other and tends to lie down, and this causes it to resurface beyonda certain speed since the hydrodynamic lift then becomes very largecompared with this body's weight in the water.

All these effects are unacceptable during operational use and it is notconceivable to limit the operational capacity of this system as afunction of the circumstances encountered, for example by reducing thespeed.

It has been attempted to improve this system by fitting it with a tailassembly, articulated to the same point of articulation as the stirrup202 and which makes it possible to stabilize this undesirable motion toa certain extent. The cable 104 enabling the linear array to be towed isthen fastened to the rear of this tail assembly. Unfortunately, whenextracting from the water during the operations for recovering the wholethe tail assembly tends to rise up, and in order to avoid thisundesirable effect it is expedient to motorize the articulation betweenthe tail assembly and the body, so as to align the latter with the tailassembly before extraction from the water. This motorization naturallycomplicates the device and increases its cost. It is furthermore onlyrelatively effective and it is observed that the placing of such a bodyinto the water and its recovery present major difficulties. In fact,when part of the body begins to emerge, the drag of the submerged partbecomes predominant and tends to make the device as a whole tip up. Themotions can then no longer be controlled, all the more so sincedisturbances due to the wake from the boat and to the wash from thepropellers are added to the disturbances due to the transit at theair/water interface.

Furthermore the location for storing this device in the towboat outsideof operational periods is of limited dimensions. In order to be able tostow the vehicle in this location, it is then expedient to install asystem for tipping the vehicle up at the time of anchorage or recovery,so as to be able to stow it in a position which is compatible with thelimited height available in the storage location. The means to beimplemented are heavy and expensive and their use is risky anddangerous.

SUMMARY OF THE INVENTION

To alleviate these drawbacks, the invention proposes a towed acoustictransmitter, of the type comprising a vertical faired acoustic array anda towing hook, mainly characterized in that it comprises a ballasted andfaired fore body joined to the array by a girder of small cross-section,in that the respective masses of the array and of the body are such thatthe centre of gravity of the whole is situated near the junction of thegirder and the fore body and beneath this girder, and in that the towinghook is articulated to a swivel joint situated on the upper part of thegirder above this centre of gravity.

According to another characteristic, the girder is fixed substantiallyat the centre of the faired array and the array also supports horizontalailerons.

According to another characteristic, the transmitter furthermorecomprises means for towing a linear acoustic array fixed to the rear ofthe faired acoustic array along the alignment of the said girder.

According to another characteristic, the transmitter furthermorecomprises horizontal depressor ailerons fixed to the girder to the rearof the point of fixing of the towing hook, so as to obtain a negativedownlift effect tending to submerge the transmitter under the action ofthe towing.

According to another characteristic, the faired acoustic array comprisesa set of distinct transducers included in individual fairings, thesefairings being joined together by vertical fixed planes-of very thincross-section.

According to another characteristic, the faired acoustic array is hollowand comprises means enabling seawater to fill these hollow parts duringsubmersion and to escape during recovery.

According to another characteristic, the towing hook and the girder aredevised so as to enable the said transmitter to be fastened in asuspended position from a saddle situated on the transmitter anchorageand capture system.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will emerge clearly inthe following description presented by way of non-limiting example inconjunction with the appended figures which represent:

FIG. 1, a diagrammatic view of the general system for towing a linearacoustic array by way of a towed acoustic transmitter;

FIG. 2, a view of a towed acoustic transmitter according to the knownart;

FIGS. 3 and 4, side and plan views of a towed acoustic transmitteraccording to the invention;

FIG. 5, a plan view of a variant of the invention; and

FIG. 6, a view of the array of another variant of the invention,

FIG. 7, a plan view of a sub-variant of the variant of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The transmitter vehicle according to the invention, represented in FIGS.3 and 4, comprises a faired acoustic transmission array 303 whosehorizontal cross-section is ovoid so as to adopt a streamlined profileof minimum resistance and minimum lift. This array is vertical and formsa rear fin of the towed vehicle.

This array is fixed at its middle to a girder 310 of small cross-sectionwhich extends rearward via a tie 311 making it possible to fasten thecable 105 for hauling the linear acoustic receiving array.

The acoustic array 303 furthermore includes two horizontal ailerons 312which are fixed perpendicularly to its surface at the centre of thelatter, and hence at the level of the girder 310. These ailerons formthe horizontal plane of the tail assembly of the towed vehicle and, inconjunction with the action of the faired array, make it possible tostabilize this vehicle.

The fore part of the vehicle is formed by a faired ballast 313 which isfixed to the fore part of the girder 310. In the figure this fairedballast is represented by a hefty sphere surrounded by a streamlinedfairing, but it may very well consist in practice of the fairing and itscontents as a whole. Advantageously this ballast will be formed by theelectronic transmission units making it possible to power thetransducers of the array 303 by way of connection cables passing throughthe girder 310, which will then be hollow. The heavy towing cable 101 isfastened to a rigid arm 314, which is linked to the towed vehicle by wayof a swivel joint depicted in a simplified manner in the figure by ahorizontal pin 315, which serves as articulation between this arm andthe vehicle. This pin is fixed to the vehicle at the junction pointbetween the arm 310 and the faired ballast 313. In the embodimentdescribed in the figure, that end of the arm 314 which is fixed to thepin 315 has the shape of a fork with two prongs which are fixed to theends of this pin, and the pin itself is fixed to the towed vehicle byits central part. Any other system ensuring articulation according to ahorizontal axis perpendicular to the direction of advance of the vehiclecan be used.

Bearing in mind the masses stemming from the structure of the vehicle,the fore part of the latter is ballasted in such a way as to represent amass M1 whose resultant with the mass M2 corresponding to the whole ofthe rear part of the vehicle, that is to say to the mass of the fairedarray 303, corresponds to a mass M1+M2 applied at a centre of gravitywhich lies plumb with the pin 315 and beneath the latter when thevehicle exhibits an operating trim in which the arm 310 and the ailerons312 are horizontal.

The centre of drag, corresponding in essence to the hydrodynamic thruston the tail assembly formed by the faired array 303 and the ailerons312, will itself be situated far to the rear of this pin 315.

Under these conditions, when the vehicle is towed the forcecorresponding to the weight of the vehicle will be balanced by thehauling force by the cable 101 and the drag force corresponding to thelift of the tail assemblies and to that of the acoustic linear array105. Bearing in mind the respective arrangements of the various unitsand the forces present described above, any onset of instability tendingto deviate the longitudinal axis of the towed vehicle, demarcated by thegirder 310, away from the hauling boat towing trajectory, tends toinitiate a restoring torque which returns the vehicle to its position ofuse corresponding to the direction described above for the girder 310.

Automatic stability of the vehicle towed on its trajectory has thus beenobtained.

Moreover, with such a structure it is easy to seize the vehicle at thelevel of the girder 310, for example with the aid of a device known as afastening saddle. Under these conditions, when the vehicle is brought onboard the towboat, it is possible to fasten it to such a saddle fixed tothe mechanical anchorage and capture system, at a height sufficient toallow the faired array 303 to be accommodated between the ceiling andthe floor of the storage hangar. The vehicle will thus be manipulated inits natural position, which is itself stable as a function of theposition of its centre of gravity, and the manipulations to be performedin order to place it in the storage hangar and fix it to the retainingsaddle are reduced to the minimum and will not present any difficulties,or therefore any particular danger.

The whole formed by this vehicle and the linear array which to itfastened sic! sails at a specified depth of submersion, the value ofwhich is adjusted by means known per se. With the aim of facilitatingthe action of these means for controlling submersion, so as among otherthings to be able to attain the largest submersions, the invention alsoproposes to fit horizontal planes 316 such as represented in FIG. 5 tothe vehicle as described above which make it possible to obtain adownwards lift which is added to the weight in the water so as toincrease the submersion. The value of this downlift varies according tothe square of the speed, in the same way as the drag of the vehicle,thus making it possible to stabilize the submersion.

In this case it is possible, in order to facilitate stability during thegyration manoeuvres, to tie the receiving array 105 to the ends of theplanes 316 by way of a cable 701 forming a V whose tip is behind thetransmission array, rather than to the rear of the transmission array303. The receiving array is fixed by a boom 704 and a clevis 703 to apulley 702 which rolls inside the V formed by the cable 701. Theelectrical connections are ensured by a slack cable 705. In this way thelateral displacement of the towing point corresponds virtually to arotation about the intersection of the perpendiculars to the cable atits securing points on the planes 316. Thus the whole behaves as if thereceiving array were fastened close to the towing centre, both laterallyand horizontally.

Moreover, in certain cases the transducers of the transmission arraywhich are contained in the fairing 303 are separated by relatively largedistances compared with the dimensions of these transducers. Therefore,it is not then useful to use a monobloc fairing to enclose thetransducers as a whole. Under these conditions, the invention proposesby way of a variant, as represented in FIG. 6, to split up this fairing303 so as to obtain for example three fairings 603 to 623 enclosing thethree distinct transducers 503, 513 and 523. These fairings will havethe most streamlined shape possible and as in the case represented inthe figure will exhibit an ovoid shape. They will be held together byvertical fixed planes 601 and 602, of fairly narrow cross-section andexhibiting only very small drag. The thickness of these fixed planeswill be reduced to the minimum making it possible to obtain thenecessary rigidity while allowing internal passage of the transducerpower conductors 503 to 523.

Finally in certain cases the transducers used include relatively largehollow internal volumes which tend to give the whole positive buoyancy,this being unhelpful to the objective sought. To compensate for thisbuoyancy, it is expedient to ballast the vehicle in an additionalmanner. When this ballast is obtained by construction, it obviouslyrequires greater effort and pieces dimensioned accordingly so as to beable to manipulate the vehicle while it is being extracted from thewater and during stowage thereof in its hauling vessel. To obtainsimultaneously a sufficient weight in the water and as small as possiblea weight in the air when the vehicle has been extracted from the water,the invention furthermore proposes to devise the hollow internal partsof these transducers in such a way as to be able to introduce thereto alarge quantity of water when they are submerged. The reservoirs locatedfor this purpose inside these transducers so as to receive the waterwill be designed in such a way as not to impair the acousticcharacteristics of the transducers, for example by designing an airsheet between these reservoirs and the active part of the transducers.Furthermore the openings intended for draining the filling water whenthe vehicle leaves the water will be devised in such a way that thewater is discharged naturally by gravity.

The towed acoustic transmitter vehicle thus described allows stablesailing at depth within a large speed region which may exceed 30 knots.Manoeuvring tolerance during sailing is particularly great. Anchorageand raising of these vehicles is performed as easily as with bodies ofhorizontal shape since anchorage and capture are performed from above,and finally the stability in the wake during submersion and recovery isentirely satisfactory.

What is claimed as new and desired to be secured by Letters Patent ofthe U.S. is:
 1. Towed acoustic transmitter, of the type comprising avertical faired acoustic array and a towing hook, said towed acoustictransmitter comprising a ballasted and faired fore body joined to thearray by a girder of small cross-section, in that the respective massesof the array and of the body are such that the centre of gravity of saidtowed acoustic transmitter is situated near the junction of the girderand the fore body and beneath this girder, and wherein the towing hookis articulated to a swivel joint situated on the upper part of thegirder above this centre of gravity.
 2. Transmitter according to claim1, characterized in that the girder is fixed substantially at the centreof the faired array and in that this array also supports horizontalailerons.
 3. Transmitter according to claim 1, characterized in that itfurthermore comprises means for towing a linear acoustic array fixed tothe rear of the faired acoustic array along the alignment of the saidgirder.
 4. Transmitter according to claim 1, characterized in that itfurthermore comprises horizontal depressor ailerons fixed to the girderto the rear of the point of fixing of the towing hook, so as to obtain anegative downlift effect tending to submerge the transmitter under theaction of the towing.
 5. Transmitter according to claim 1, characterizedin that the faired acoustic array comprises a set of distincttransducers included in individual fairings, these fairings being joinedtogether by vertical fixed planes of very thin cross-section. 6.Transmitter according to claim 1, characterized in that the fairedacoustic array is hollow and comprises means enabling seawater to fillthese hollow parts during submersion and to escape during recovery. 7.Transmitter according to claim 1, characterized in that the towing hookand the girder are devised so as to enable the said transmitter to befastened in a suspended position from a saddle situated on thetransmitter anchorage and capture system.
 8. Transmitter according toclaim 4, further comprising means for towing a linear acoustic array,comprising a V-shaped cable fixed by its two ends to the ends of the thedepressor ailerons, a pulley which rolls inside the V formed by thecable, and means for fastening the linear acoustic array to the pulley.9. Transmitter according to claim 2, further comprising means for towinga linear acoustic array fixed to the rear of the faired acoustic arrayalong the alignment of the said girder.
 10. Transmitter according toclaim 2, further comprising horizontal depressor ailerons fixed to thegirder to the rear of the point of fixing of the towing hook, so as toobtain a negative downlift effect tending to submerge the transmitterunder the action of the towing.
 11. Transmitter according to claim 3,further comprising horizontal depressor ailerons fixed to the girder tothe rear of the point of fixing of the towing hook, so as to obtain anegative downlift effect tending to submerge the transmitter under theaction of the towing.
 12. Transmitter according to claim 2,characterized in that the faired acoustic array comprises a set ofdistinct transducers included in individual fairings, these fairingsbeing joined together by vertical fixed planes of very thincross-section.
 13. Transmitter according to claim 3, characterized inthat the faired acoustic array comprises a set of distinct transducersincluded in individual fairings, these fairings being joined together byvertical fixed planes of very thin cross-section.
 14. Transmitteraccording to claim 4, characterized in that the faired acoustic arraycomprises a set of distinct transducers included in individual fairings,these fairings being joined together by vertical fixed planes of verythin cross-section.
 15. Transmitter according to claim 2, characterizedin that the faired acoustic array is hollow and comprises means enablingseawater to fill these hollow parts during submersion and to escapeduring recovery.
 16. Transmitter according to claim 3, characterized inthat the faired acoustic array is hollow and comprises means enablingseawater to fill these hollow parts during submersion and to escapeduring recovery.
 17. Transmitter according to claim 4, characterized inthat the faired acoustic array is hollow and comprises means enablingseawater to fill these hollow parts during submersion and to escapeduring recovery.
 18. Transmitter according to claim 5, characterized inthat the faired acoustic array is hollow and comprises means enablingseawater to fill these hollow parts during submersion and to escapeduring recovery.
 19. Transmitter according to claim 2, characterized inthat the towing hook and the girder are devised so as to enable the saidtransmitter to be fastened in a suspended position from a saddlesituated on the transmitter anchorage and capture system. 20.Transmitter according to claim 3, characterized in that the towing hookand the girder are devised so as to enable the said transmitter to befastened in a suspended position from a saddle situated on thetransmitter anchorage and capture system.